1. Field of the Invention
This invention relates to a process for producing a non-single crystal semiconductor type photovoltaic device by a roll-to-roll system.
2. Related Background Art
Photovoltaic devices which are photoelectric conversion devices that convert sunlight into electric energy are put into wide use as public-purpose power sources for low-power supply such as in electronic calculators and wrist watches. Photovoltaic devices also attract notice as possible future substitute power generation means for petroleum fuel such as oil and coal. Photovoltaic devices utilize photovoltaic force attributable to, e.g., p-n junction of semiconductor devices. Semiconductors such as silicon absorb sunlight to produce photocarriers of electrons and holes by the aid of photon energy, and the photocarriers are taken out by differences in chemical potential at the p-n junction region.
In order to bring photovoltaic devices into practical use as electric power sources, it is important to achieve cost reduction and large-area devices, and various studies is conducted thereon. Researches are made on materials such as low-cost materials and materials with high photoelectric conversion efficiency. Such materials for photovoltaic devices may include tetrahedral type amorphous semiconductors such as amorphous silicon, amorphous silicon germanium and amorphous silicon carbide, and compound semiconductors of Groups II-VI such as CdS and Cu2S and those of Groups III-V such as GaAlAs. In particular, thin-film photovoltaic devices in which amorphous semiconductors are used in photovoltaic layers have advantages that they can provide films having larger area than single-crystal photovoltaic devices, can be formed in a small layer thickness and can be deposited on any desired substrate material; thus they are regarded as promising.
However, in order to put such amorphous semiconductor type photovoltaic devices into practical use as electric power sources, it has been a subject for study to improve photoelectric conversion efficiency and improve reliability.
As a means for improving the photoelectric conversion efficiency of the photovoltaic devices making use of amorphous semiconductors, various methods are available. For example, with regard to a photovoltaic device that utilizes a p-i-n type semiconductor junction, a p-type semiconductor layer, an i-type semiconductor layer, an n-type semiconductor layer, a transparent electrode and a back surface electrode which constitute the device must be improved in characteristics for each layer.
As another method for improving photoelectric conversion efficiency of photovoltaic devices, U.S. Pat. No. 2,949,498 discloses the use of what is called a stacked cell, in which photovoltaic devices having a certain unit device structure are superposed in plurality. This stacked cell makes use of p-n junction crystal semiconductors. Its concept is common to both amorphous and crystalline and is to make sunlight spectra absorb efficiently through photovoltaic devices having different band gaps and make open-circuit voltage (Voc) higher so that electricity generation efficiency can be improved.
In the stacked cell, constituent devices having different band gaps are superposed in plurality, and sunlight rays are absorbed efficiently at every part of their spectra so that photoelectric conversion efficiency can be improved. The cell is so designed that what is called the bottom layer positioned beneath what is called the top layer has a narrower band gap than the band gap of the top layer positioned on the light-incident side of the superposed constituent devices.
Meanwhile, Y. Hamakawa, H. Okamoto and Y. Nitta report what is called a cascade type cell, in which amorphous silicon layers having the same band gaps are superposed in multi-layer in such a way that no insulating layer is provided between photovoltaic devices so that the open-circuit voltage (Voc) of the whole device can be made higher. This is a method in which unit devices made of amorphous silicon materials having the same band gaps are superposed.
In the case of such stacked cells, too, like the case of single-layer cells (single cells), in order to improve photoelectric conversion efficiency, characteristics must be improved for each layer of the p-type semiconductor layer, i-type semiconductor layer, n-type semiconductor layer, transparent electrode and back electrode which constitute the photovoltaic device.
For example, in the case of the photoactivation layer, i-type semiconductor layer, it is very important to make band-gap internal levels (localized levels) as low as possible to improve transport performance of photocarriers.
With regard to what is called doped layers such as the p-type semiconductor layer and n-type semiconductor layer, it is first required that their activated acceptors or donors are in high density and can be activated at a small energy. This makes diffusion potential (built-in potential) large when a p-i-n type junction is formed and enhances the open-circuit voltage (Voc) of the photovoltaic device, bringing about an improvement in photoelectric conversion efficiency.
It is second required that the doped layers, which basically do not contribute to the generation of photocurrent, do not obstruct, as far as possible, the light entering the photocurrent-generating i-type semiconductor layer. Accordingly, in order to make the doped layers absorb less light, it is important to make their optical band gaps wide and to form them in small layer thickness.
Materials for doped layers having such characteristics include, e.g., Group IV semiconductor materials such as Si, SiC, SiN and SiO, and those having amorphous or microcrystalline form have been studied. In particular, Group IV semiconductor alloy materials having a wide optical band gap have been considered preferable because of their small absorption coefficient, and microcrystalline or polycrystalline semiconductor materials are preferred, because of their small absorption coefficient and small activation energy.
However, significant lowering of carrier transport performance and fill factor (FF) has occurred which is ascribable to lattice matching and junction interfacial levels between the i-type semiconductor layer and the microcrystalline or polycrystalline p-type semiconductor layer, and its improvement has been a subject for study.
Methods for solving such problems are under study. As an example thereof, U.S. Pat. Nos. 4,254,429 and No. 4,377,723 disclose a method in which what is called a buffer layer(s) is/are provided at the junction interface(s) between the p-type semiconductor layer and/or n-type semiconductor layer and the i-type semiconductor layer. At the junction interface between the p-type semiconductor layer or n-type semiconductor layer and the i-type semiconductor layer, the former being formed of amorphous silicon and the latter being formed of amorphous silicon germanium, many midgap levels are produced because of differences in lattice constant. Hence, they serve as the center of recombination at the junction interface to make the lifetime of carriers short. Such a buffer layer is formed so that by the use of the buffer layer the band-gap internal levels can be reduced and the carrier transport performance is not damaged, thereby bringing about an improvement in characteristics.
Now, as a process for producing photovoltaic devices by forming semiconductor functional deposited films continuously on a substrate, a process is known in which independent film-forming chambers for forming all kinds of semiconductor layers are provided. The respective film-forming chambers are connected through gate valves by a load-lock system, and the substrate is moved successively to the respective film-forming chambers to form thereon all kinds of semiconductors.
As a process which can improve mass productivity greatly, U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD (chemical vapor deposition) process employing a roll-to-roll system. According to this process, a continuous belt-like substrate is used as a substrate, and the substrate is transported continuously in its lengthwise direction while depositing and forming semiconductor layers with any necessary conductivity types in a plurality of glow discharge regions to form continuously devices having semiconductor junctions.
A deposited film forming apparatus of the above roll-to-roll system is constituted of a belt-like substrate wind-off chamber and a wind-up chamber which are provided at both ends, respectively, and provided between them deposited-film-forming chambers for forming a plurality of semiconductor layers by plasma CVD, which are connected through gas gates. Into the gas gates, a scavenging gas such as H2 gas is introduced to form pressure barriers against adjoining deposited-film-forming chambers so that the gas can be prevented from diffusing across the chambers. This is characteristic of the roll-to-roll system film-forming apparatus. Materials gases are fed to each deposited-film-forming chamber, and high-frequency or microwave power is applied thereto to cause discharge to take place in the discharge space. Each deposited-film-forming chamber also has an evacuation means and a pressure control valve so that its inside can be maintained at a vacuum state with a certain pressure.
In actual film formation, the continuous belt-like substrate is stretched over the wind-off chamber and the wind-up chamber, and semiconductor layers can be deposited and formed successively in the discharge spaces of the deposited-film-forming chambers while feeding and moving forward the substrate continuously.
In the roll-to-roll system, in view of its film-forming process, a film formed on the belt-like substrate has principally no difference in the transport direction of the belt-like substrate. In the width direction of the belt-like substrate, however, the film has a boundary condition at its edge areas which is quite different from its center area, especially when a high-frequency power is used as excitation energy and a parallel-plate electrode is used. This may cause a lowering of the density of excitation energy. Also, when a microcrystalline material is used in the buffer layer, a high feeding energy is required in order to form microcrystals, and hence a serious problem may occur with respect to the distribution of crystallinity in the width direction of the belt-like substrate. Such a difference in crystallinity brings about a difference in band gaps and activation energy of films, and hence may hinder the formation of desired junctions to bring about an increase in series resistance of the photovoltaic device, thereby resulting in a decrease in photoelectric conversion efficiency because of a lowering of the fill factor.
An object of the present invention is to provide, in such a roll-to-roll system, a process for producing a photovoltaic device having a uniform photoelectric conversion efficiency, which is attributable to the formation of a microcrystalline semiconductor layer having lower characteristics distribution in the width direction of a belt-like substrate.
The present invention provides a process for producing a photovoltaic device, comprising the step of forming a semiconductor layer comprising a non-single crystal first-conductivity type semiconductor layer, an amorphous i-type semiconductor layer, a microcrystalline i-type semiconductor layer and a microcrystalline second-conductivity type semiconductor layer, on a belt-like substrate while transporting the belt-like substrate continuously in its lengthwise direction;
depositing a microcrystalline i-type semiconductor layer being accomplished by introducing a film-forming gas into a discharge space one face of which is formed by the belt-like substrate and simultaneously applying a high-frequency power from a parallel-plate electrode facing the belt-like substrate, to cause plasma in the discharge space to form a deposited film continuously on the surface of the belt-like substrate;
wherein an area of the parallel-plate electrode is represented by S; a width of the discharge space in its direction perpendicular to the transport direction of the belt-like substrate, by Ws; a width of a region formed by the parallel-plate electrode together with its surrounding insulating region, in its direction perpendicular to the transport direction of the belt-like substrate, by Wc; a width of the belt-like substrate in the direction perpendicular to its transport, by Wk; a distance between the parallel-plate electrode and the belt-like substrate, by h; a power density at which a crystal fraction begins to saturate at predetermined substrate temperature, material gas flow rate and pressure, by Pd; and the high-frequency power, by P; and wherein
xe2x80x832h/(Wsxe2x88x92Wc)xe2x89xa72.5, (Ws/h)xc3x972(Wsxe2x88x92Wk)/[4h+(Wsxe2x88x92Wc)]xe2x89xa710, and Pxe2x89xa7(10/8)xc3x97Pdxc3x97S.
In a preferred embodiment of the present invention, a value of Wc/h is 10 or more. Also, the belt-like substrate used in the present invention may preferably be electrically conductive.